1. Field of the Invention
The present invention relates to an aluminum product having a metal diffusion layer on the surface thereof by a metal diffusion treatment; a process for producing the same; and a paste for metal diffusion treatment.
2. Description of the Related Art
Aluminum forms intermetallic compounds such as CrAl.sub.7, CuAl.sub.2, Mg.sub.2 AL.sub.3, TiAl.sub.3, NiAl.sub.3 by combining with chromium, copper magnesium, titanium and nickel respectively. These intermetallic compounds have been known that they are hard and superior in heat resistance.
If two kinds of metal plates are kept at high temperature being contacted, metallic atoms at one of each metallic plate on the contact surface thereof diffuse to a surface portion of the other metallic plate so that an intermetallic compound is theoretically formed on each of metallic plates by this mutual diffusion. However, at around the temperature of 550.degree. C., a mutual diffusion of aluminum and the other metal has extremely small mutual diffusion speed ranging from 10.sup.-12 to 10.sup.-18 m.sup.2 /sec. so that it is almost impossible to form a metal diffusion layer only by mutual diffusion practically. Furthermore, it has been tried that the other metallic layer is formed on the surface of the aluminum product by thermal spraying, however, it has not been obtained that dissimilar metals diffuse inclinatorily not less than 1 .mu.m from the surface of the aluminum product and also, the aluminum product includes a metal diffusion layer, on the surface thereof, comprising a diffusing metal being not less than 1.0% by weight.
On the other hand, the present inventors disclose the following method in Japanese Patent Application (TOKUGAN) No. 7-100184: getting a nitriding agent into contact with at least on one part of the surface of the aluminum product, wherein the nitriding agent comprises a metal powder including at least one selected from the group consisting of titanium, chromium, silicon, iron, manganese, nickel, vanadium, tantalum, magnesium, boron and zirconium as a major component; and in this state, nitriding the surface of the aluminum product by an ambient gas comprising substantially gaseous nitrogen at a temperature of a melting point of the aluminum product or less. The purpose of this procedure is to obtain a thick nitriding layer on the surface of the aluminum product. In this procedure, the metal powder used as the nitriding agent is subjected to nitriding and becomes a nitrided metal, however it is not diffused into the inside of aluminum.
Furthermore, the present inventors disclose the following method in Japanese Unexamined Patent Publication (KOKAI) No. 7-166321: getting a nitriding agent which is composed of an aluminum powder into contact with at least one part of the surface of the aluminum product; and in this state, nitriding the surface of the aluminum product by an ambient gas comprising substantially gaseous nitrogen at a temperature of a melting point of the aluminum product or less. In this method, a pure aluminum powder or an aluminum product including magnesium a little is recommended to be used as nitriding agent and the purpose of this method is to obtain the nitriding layer at the inside of aluminum.
Furthermore, the present inventors disclose the following product in Japanese Unexamined Patent Publication (KOKAI) No. 7-292454: a case treated aluminum product comprising an aluminum matrix, a diffusion layer formed by diffusion of nitrogen in the aluminum-matrix which is formed at the surface portion of the aluminum matrix, a sintered layer which is formed by sintering of mainly a nitrided aluminum powder which is formed on the upper surface of the diffused layer. Also, in this publication, the present inventors, at the same time, disclose the case treated aluminum product which exists by making compound by conducting diffusion and penetration of nitrogen and other elements except for nitrogen in the diffused layer, however the treatment method thereof is poor and in the treatment method in which titanium powder is used, the amount of titanium which exists in the diffusion layer is extremely little, and the result thereof is not satisfactory in actual use.